The invention relates to a method and device for monitoring the wire-stitching on print products in a wire-stitching machine, wherein measuring devices are provided for testing the wire staple quality.
The technique of stapling together print products by means of wire staples in a wire-stitching apparatus is known. Wire-stitching machines typically comprise a stitching head and a wire-bending device for realizing the stitching operation. The operation involves supplying a wire, cutting the blank, forming the staple, pushing the staple through the product to be stapled, and bending the two staple legs.
Methods and devices are known which can be used to test for the existence of a wire staple on a product, thus making it possible to remove a product that is missing a wire staple. The testing can be realized, for example, by means of a metal detector which is arranged downstream from the wire-stitching apparatus. Each passing wire staple triggers an impulse. A missing impulse therefore indicates a product with a missing wire staple. Furthermore, European Patent Document EP 0 205 144 teaches an apparatus wherein a missing wire staple is detected by means of a sensor arranged on a stitching machine, wherein the sensor comprises one of an approximation switch or an optical sensor.
However, the above-mentioned methods and devices can only be used to detect the presence, and not the quality, of the wire-stitching. Thus, wire staples which are defective, for example those that have an outward-projecting leg, are nonetheless indicated as being present. Wire-stitching defects of this type are highly undesirable because they can result in injuries to the users and/or readers of such a print product. For that reason, numerous measures have already been proposed for detecting such defective stitching operations and for removing the corresponding print products. Thus, a device for monitoring the stitching of products is known from European patent document EP 1 029 643 A, which is co-owned by the assignee of the present application, wherein the wire-stitching machine is provided with measuring devices for detecting changes in the condition of the bending device or stitching head. For example, these devices use wire strain gauges to detect the force generated at the bending device during the forming of the wire staple. If this force deviates from a predetermined value, it is assumed that the wire-stitching is defective and the product is accordingly removed via the machine control.